Apatite compounds, particularly hydroxyapatite, have excellent biological affinity because of their similarity to the inorganic component constituting bones and teeth of living organisms. Taking advantage of their biological affinity, they have been utilized as artificial implant ceramics for living organisms in structure such as artificial tooth roots, artificial bones, artificial joints, etc. However, because apatite such as hydroxyapatite is inferior in mechanical strength in comparison to living bones, it cannot be utilized as a member on which a high load is borne.
Various approaches have been investigated for incorporating apatite into a composite material with metals or high-strength ceramics (as disclosed in The International Journal of Oral & Maxillofacial Implants, vol, 2, 111 (1987)). Among various methods for producing such a composite material, a thermal spraying method has been intensely investigated in view of the uniformity of adhesion strength and the affinity of the composite with living organisms.
As a thermal spraying method for ceramics, methods utilizing plasma as a heat source have been generally employed. It has been studied in this field of art that apatite is thermal-sprayed using a plasma as a heat source (as disclosed, e.g., in JP-A-62-34558, JP-A-62-34559, JP-A-62-34566, JP-A-53-28997, JP-A-52-82893 and Journal of Biomedical Materials Research, vol. 21, 1375 (1987)). However, since apatite suffers from high temperatures of 10,000.degree. C. or more during plasma spraying, undesirable decomposition products such as tricalcium phosphate and calcium oxide are often formed in the final products, and/or sufficient adhesion strength cannot be obtained even though maintaining of adhesion of the thus-formed apatite layer is highly required.
The use of a flame spraying method to alumina is disclosed in JP-A-58-12651, but the flame spraying of apatite has not yet been established.